•A novel framework for solving irregular PDEs using Deep Neural Networks (DNNs) is developed, extended, and analyzed.•The framework demonstrates phenomenal ease of prototyping PDE solutions.•For the ...first time, irregular shock solutions to a classical fluid problem are analyzed, and compared against finite volume and finite element solutions, demonstrating good accuracy and stability behaviors.•The method demonstrates novel and remarkable computational scalability in parameter exploration.•A novel data-enrichment algorithm for incorporating experimental data into the DNN/PDE framework is presented, which is used to reveal hidden underlying physical subsystems observed in the experiment, but unaccounted for in the theoretical model.
Recent work on solving partial differential equations (PDEs) with deep neural networks (DNNs) is presented. The paper reviews and extends some of these methods while carefully analyzing a fundamental feature in numerical PDEs and nonlinear analysis: irregular solutions. First, the Sod shock tube solution to the compressible Euler equations is discussed and analyzed. This analysis includes a comparison of a DNN-based approach with conventional finite element and finite volume methods, and demonstrates that the DNN is competitive in terms of degrees of freedom required for a given accuracy. Further, the DNN-based approach is extended to consider performance improvements and simultaneous parameter space exploration. Next, a shock solution to compressible magnetohydrodynamics (MHD) is solved for, and used in a scenario where experimental data is utilized to enhance a PDE system that is a priori insufficient to validate against the observed/experimental data. This is accomplished by enriching the model PDE system with source terms that are then inferred via supervised training with synthetic experimental data. The resulting DNN framework for PDEs enables straightforward system prototyping and natural integration of large data sets (be they synthetic or experimental), all while simultaneously enabling single-pass exploration of an entire parameter space.
Recent work suggests that the first generation of stars, the so-called Population III (Pop III), could have formed primarily in binaries or as members of small multiple systems. Here we investigate ...the impact of X-ray feedback from high-mass X-ray binaries (HMXBs) left behind in stellar binary systems after the primary forms a black hole (BH), accreting gas at a high rate from the companion, a process that is thought to be favoured at the low metallicities characteristic of high-redshift gas. Thanks to their large mean free path, X-rays are capable of pre-ionizing and pre-heating the gas in the intergalactic medium (IGM) and in haloes long before the reionization of the Universe is complete, and thus could have strongly affected the formation of subsequent generations of stars as well as reionization. We have carried out zoomed hydrodynamical cosmological simulations of minihaloes, accounting for the formation of Pop III stars and their collapse into BHs and HMXBs, and the associated radiation–hydrodynamic feedback from ultraviolet and X-ray photons. We find no strong net feedback from HMXBs on the simulated star formation history. On the other hand, the pre-heating of the IGM by HMXBs leads to a strong suppression of small-scale structures and significantly lowers the recombination rate in the IGM, thus yielding a net positive feedback on reionization. We further show that X-ray feedback from HMXBs can augment the ionizing feedback from the Pop III progenitor stars to suppress gas accretion on to the first BHs, limiting their growth into supermassive BHs. Finally, we show that X-ray ionization by HMXBs leaves distinct signatures in the properties of the high-redshift hydrogen that may be probed in upcoming observations of the redshifted 21 cm spin-flip line.
We investigate magnetic reconnection and particle acceleration in relativistic pair plasmas with three-dimensional particle-in-cell simulations of a kinetic-scale current sheet in a periodic ...geometry. We include a guide field that introduces an inclination between the reconnecting field lines and explore outside-of-the-current sheet magnetizations that are significantly below those considered by other authors carrying out similar calculations. Thus, our simulations probe the transitional regime in which the magnetic and plasma pressures are of the same order of magnitude. The tearing instability is the dominant mode in the current sheet for all guide field strengths, while the linear kink mode is less important even without the guide field, except in the lower magnetization case. Oblique modes seem to be suppressed entirely. In its nonlinear evolution, the reconnection layer develops a network of interconnected and interacting magnetic flux ropes. As smaller flux ropes merge into larger ones, the reconnection layer evolves toward a three-dimensional, disordered state in which the resulting flux rope segments contain magnetic substructure on plasma skin depth scales. Embedded in the flux ropes, we detect spatially and temporally intermittent sites of dissipation reflected in peaks in the parallel electric field. Magnetic dissipation and particle acceleration persist until the end of the simulations, with simulations with higher magnetization and lower guide field strength exhibiting greater and faster energy conversion and particle energization. At the end of our largest simulation, the particle energy spectrum attains a tail extending to high Lorentz factors that is best modeled with a combination of two additional thermal components. We confirm that the primary energization mechanism is acceleration by the electric field in the X-line region. The highest-energy positrons (electrons) are moderately beamed with median angles ~30degrees-40degrees relative to (the opposite of) the direction of the initial current density, but we speculate that reconnection in more highly magnetized plasmas would give rise to stronger beaming. Finally, we discuss the implications of our results for macroscopic reconnection sites, and which of our results may be expected to hold in systems with higher magnetizations.
We present a two-dimensional grid-based hydrodynamic simulation of a thin, viscous, locally isothermal corotating disk orbiting an equal-mass Newtonian binary point mass on a fixed circular orbit. We ...study the structure of the disk after multiple viscous times. The binary maintains a central hole in the viscously relaxed disk with radius equal to about twice the binary semimajor axis. Disk surface density within the hole is reduced by orders of magnitude relative to the density in the disk bulk. The inner truncation of the disk resembles the clearing of a gap in a protoplanetary disk. An initially circular disk becomes elliptical and then eccentric. Disturbances in the disk contain a component that is stationary in the rotating frame in which the binary is at rest; this component is a two-armed spiral density wave. We measure the distribution of the binary torque in the disk and find that the strongest positive torque is exerted inside the central low-density hole. We make connection with the linear theory of disk forcing at outer Lindblad resonances (OLRs) and find that the measured torque density distribution is consistent with forcing at the 3:2 (m = 2) OLR, well within the central hole. We also measure the time dependence of the rate at which gas accretes across the hole and find quasi-periodic structure. We discuss implications for variability and detection of active galactic nuclei containing a binary massive black hole.
We investigate how radiative feedback from the first stars affects the assembly of the first dwarf galaxies. To this end, we perform cosmological zoomed smoothed particle hydrodynamics simulations of ...a dwarf galaxy assembling inside a halo reaching a virial mass ~10sup 9 M at z = 10. The simulations follow the non-equilibrium chemistry and cooling of primordial gas and the subsequent conversion of the cool dense gas into massive metal-free stars. To quantify the radiative feedback, we compare a simulation in which stars emit both molecular hydrogen dissociating and hydrogen/helium ionizing radiation with a simulation in which stars emit only molecular hydrogen dissociating radiation, and further with a simulation in which stars remain dark. Our conclusions regarding the structure and observability of the first galaxies are subject to our neglect of feedback from supernovae and chemical enrichment as well as to statistical uncertainties implied by the limited number of galaxies in our simulations.
We present results from three cosmological simulations, only differing in gas metallicity, that focus on the impact of metal fine-structure line cooling on stellar cluster formation in a ...high-redshift atomic cooling halo. Sink particles allow the process of gas hydrodynamics and accretion on to cluster stars to be followed for ∼4 Myr corresponding to multiple local free-fall times. At metallicities at least 10−3 Z⊙, gas is able to reach the cosmic microwave background temperature floor and fragment pervasively resulting in a stellar cluster of size ∼1 pc and total mass ∼1000 M⊙. The masses of individual sink particles vary, but are typically ∼100 M⊙, consistent with the Jeans mass at T
CMB, though some solar mass fragments are also produced. Below 10−4 Z⊙, fragmentation is strongly suppressed on scales greater than 0.01 pc and total stellar mass is lower by a factor of ∼3 than in the higher metallicity simulations. The sink particle accretion rates, and thus their masses, are determined by the mass of the gravitationally unstable gas cloud and prolonged gas accretion over many Myr, exhibiting features of both monolithic collapse and competitive accretion. Even considering possible dust-induced fragmentation that may occur at higher densities, the formation of a bona fide stellar cluster seems to require metal line cooling and metallicities of at least ∼10−3 Z⊙.
The Lyman α signature of the first galaxies Smith, Aaron; Safranek-Shrader, Chalence; Bromm, Volker ...
Monthly Notices of the Royal Astronomical Society,
06/2015, Letnik:
449, Številka:
4
Journal Article
Recenzirano
Odprti dostop
We present the Cosmic Lyman α Transfer code, a massively parallel Monte Carlo radiative transfer code, to simulate Lyman α (Lyα) resonant scattering through neutral hydrogen as a probe of the first ...galaxies. We explore the interaction of centrally produced Lyα radiation with the host galactic environment. Lyα photons emitted from the luminous starburst region escape with characteristic features in the line profile depending on the density distribution, ionization structure, and bulk velocity fields. For example, anisotropic ionization exhibits a tall peak close to line centre with a skewed tail that drops off gradually. Idealized models of first galaxies explore the effect of mass, anisotropic H ii regions, and radiation pressure driven winds on Lyα observables. We employ mesh refinement to resolve critical structures. We also post-process an ab initio cosmological simulation and examine images captured at various distances within the 1 Mpc3 comoving volume. Finally, we discuss the emergent spectra and surface brightness profiles of these objects in the context of high-z observations. The first galaxies will likely be observed through the red damping wing of the Lyα line. Observations will be biased towards galaxies with an intrinsic red peak located far from line centre that reside in extensive H ii super bubbles, which allows Hubble flow to sufficiently redshift photons away from line centre and facilitate transmission through the intergalactic medium. Even with gravitational lensing to boost the luminosity this preliminary work indicates that Lyα emission from stellar clusters within haloes of M
vir < 109 M⊙ is generally too faint to be detected by the James Webb Space Telescope.
Towards ab initio extremely metal-poor stars Ritter, Jeremy S; Safranek-Shrader, Chalence; Milosavljevic, Milo ...
Monthly notices of the Royal Astronomical Society,
12/2016, Letnik:
463, Številka:
3
Journal Article
Recenzirano
Extremely metal-poor stars have been the focus of much recent attention owing to the expectation that their chemical abundances can shed light on the metal and dust yields of the earliest supernovae. ...We present our most realistic simulation to date of the astrophysical pathway to the first metal-enriched stars. We simulate the radiative and supernova hydrodynamic feedback of a 60 ... Population III star starting from cosmological initial conditions realizing Gaussian density fluctuations. We follow the gravitational hydrodynamics of the supernova remnant at high spatial resolution through its freely expanding, adiabatic, and radiative phases, until gas, now metal-enriched, has resumed runaway gravitational collapse. Our findings are surprising: while the Population III progenitor exploded with a low energy of 10 super( 51) erg and injected an ample metal mass of 6 ..., the first cloud to collapse after the supernova explosion is a dense surviving primordial cloud on which the supernova blast wave deposited metals only superficially, in a thin, unresolved layer. The first metal-enriched stars can form at a very low metallicity, of only ..., and can inherit the parent cloud's highly elliptical, radially extended orbit in the dark matter gravitational potential. (ProQuest: ... denotes formulae/symbols omitted.)
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